This invention relates to an improved pyrohydrolysis process for the recovery of fluorine, sodium and aluminum values from spent and waste materials of electrolytic aluminum reduction systems.
More particularly, the present invention provides an improved pyrohydrolysis system for processing without agglomeration both the coarse and fine materials generated in the feed preparation step which precedes pyrohydrolysis with simultaneous reduction in the dust carry-over and minimization of backreactions during the course of pyrohydrolysis.
It is known that pyrohydrolysis of the spent and waste materials of electrolytic aluminum reduction systems results in the recovery of aluminum, sodium and fluorine values. Thus, U.S. Pat. No. 4,113,832 (N. Bell et al) describes a fluidized bed process wherein the spent and waste materials, such as spent potlinings, channel and trench cleanings, as well as spent alumina from dry scrubber systems, are processed in a pyrohydrolysis unit and as a result of pyrohydrolysis sodium fluoride, aluminum fluoride, HF and sodium aluminate clinker are recovered. As shown in this patent, the fine fraction of less than 1-2 mm in size, which is generated in the comminuting stage of pyrohydrolysis feed preparation, is subjected to a shaping step in order to avoid introduction of excessively fine materials in the reactor. The fine materials, unless agglomerated, can provide difficult operating conditions in the reactor and can also generate dust separation problems in the equipment associated with the pyrohydrolysis reactor. Thus, for efficient and well-controlled operation of the pyrohydrolysis process in the fluidized bed reactor of this patent, agglomeration of the finely divided feed material is a requirement.
In copending applications Ser. No. 910,416 (May 30, 1978), an improved process for pyrohydrolyzing the spent and waste materials of electrolytic aluminum reduction systems is disclosed. In accordance with the process shown in this copending application, the pyrohydrolysis process is controlled in such a manner as to eliminate the carry-over of sodium fluoride in the offgas recovery system, thus producing essentially only HF, which can be recovered as such or employed for the manufacture of a high grade aluminum fluoride (&gt;90% AlF.sub.3 content). To achieve the conversion of the NaF constituent of the pyrohydrolysis offgas to HF, the process disclosed in Ser. No. 910,416 employs a two-stage fluidized bed reactor. In the first stage, the feed is charged to the reactor bottom portion, where it is fluidized and simultaneously pyrohydrolyzed by introduction of water. The generated offgases, containing steam, volatilized fluorine and sodium values, are then contacted in the reactor freeboard area with a finely divided source of Al.sub.2 O.sub.3. This contact in essence provides an "extended" reaction zone or second stage where conversion of the volatilized sodium values to the desired HF and sodium aluminate (Na.sub.2 O.xAl.sub.2 O.sub.3) takes place. From the "extended" zone, the offgas is conducted through a dust separation and recovery system, the dust- and essentially NaF-free HF is further processed, while the recovered dust is partly reintroduced into the "extended" zone and partly recycled to the feed preparation stage for utilization of its unreacted alumina content. Since the particle size of the finely divided source of alumina is generally selected to be less than 1 mm for several reasons, including provision for reactive surface, any material recycled to the feed preparation stage is too small for direct charging to the fluidization zone. Consequently, any alumina source recycled to the feed preparation stage has to be agglomerated, preferably together with the fine fraction of the comminuted spent and waste materials.
Thus, it can be seen that the recovery of valuable components from spent and waste materials of electrolytic aluminum reduction systems by pyrohydrolysis in the prior art methods involved an agglomeration step of the finely divided feed materials. This agglomeration step, due to the great variety of feed materials and their differing physical behavior under known agglomeration conditions, poses a serious technical problem, aside from the costly and time-consuming nature of agglomeration operations.
The present invention is directed to the elimination of the previously required agglomeration operation and at the same time it provides a process in which the sodium to aluminum ratio in the clinker can be readily controlled under conditions which assure conversion of the volatilized sodium values to the desired HF with practically no backreaction occuring. These aims are achieved by providing a staged pyrohydrolysis process wherein the coarse fraction of the comminuted feed is charged to a dense phase fluidized reaction zone of the reactor together with a portion of the fines, the remaining fine fraction of the feed is introduced in the area above the dense bed of the reactor, where it becomes fluidized in a dilute phase and the volatile reaction products from both of these stages are then reacted with a finely divided source of alumina in a third stage, comprising a dilute phase fluidized bed reactor, to convert residual sodium values to HF and Na.sub.2 O.xAl.sub.2 O .sub.3. From the first dilute phase reactor, e.g., the second stage, the dust-containing offgases prior to being charged to the third stage are subjected to a dust separation step and a major portion of the recovered dust is recycled to the second stage, and the residue of the dust is combined with the clinker exiting from the dense phase bed. The offgas from the third stage, e.g., the second dilute phase fluidized bed reactor, are subjected to dust separation. The essentially dust-free gas is used for HF recovery, while a major portion of the collected dust is recycled to the third stage and the remaining portion is charged to the first dilute phase bed. Addition of this material, having relatively low sodium to aluminum ratio, to the first dilute phase bed in a controlled amount, allows control of the final sodium to aluminum ratio in the produced clinker. The novel combination of these operational steps provides a pyrohydrolysis process wherein the clinker has a controlled Na:Al mole ratio to assure optimum leachability; a minimized dust load in the offgas leaving for HF utilization, as well as an essentially Na-free HF content.